2012 Proceedings - International Tissue Elasticity Conference

74
Session MIP–2: Methods for Imaging Elastic Tissue Properties – II
Thursday, October 4 10:30A – 12:00P
051 SHEAR WAVE IMAGING WITH A CONVENTIONAL SCANNER: THE PASSIVE ELASTOGRAPHY
APPROACH.
Stefan Catheline 1 , Rémi Souchon 1 and Jean–Yves Chapelon 1 .
1 INSERM u1032, University of Lyon, Lyon, FRANCE.
Background: In order to be quantitative, static elastography uses strong assumptions on the stress field
homogeneity. In practical in vivo situations, the stress field cannot be controlled, thus static elastography
results in a strain imaging technique not systematically related to the medium elasticity. However, one of
its crucial advantages resides in its simple implementation on commercial ultrasonic scanners with a
typical 50Hz maximum frame rate. In this presentation, we propose a solution to control the stress fields
while keeping a standard frame rate imaging (50Hz).
Methods: The idea is to replace the homogeneous stress assumption by an equipartitioned field
assumption. Indeed, a homogeneously distributed shear wave diffuse field is used with a time reversal
approach to create a point force and, from the resulting strain field, compute a shear elasticity mapping.
It is thus a mix between shear wave imaging and strain imaging; the stress is piloted by the shear wave
equation and, more precisely, by the equipartition principle of energy density on one hand, the elasticity
emerges from the Hooke’s law on the other hand. Experiments on a CIRS® phantom containing three
inclusions, 5mm diameter with elastic moduli ranging from 14 to 80kPa, were performed. The experiment
is as follows: in the first step, a diffuse wave field is created inside the sample by random finger impacts
on the surface for 20 seconds. The 2D displacement field is then measured inside the soft solid using
speckle tracking algorithms developed in elastography. It involves a 192–channel array working at 12MHz
with a repetition frequency of 50Hz. In the second step, the displacement at one point chosen as a virtual
source is correlated to the other points of the image in order to compute a time reversal field in the
computer. The under–sampling of displacements in time does not alter the spatial coherence.
Quantitative elasticity images are retrieved from the computed time reversal field using slightly different
approaches: wavelength, displacement and strain estimations. These approaches are compared and their
validation tested through finite difference simulations and experiments. A first in vivo test on the thyroid
of a healthy volunteer will be discussed.
Results: Normalised shear elasticity maps are retreived from a diffuse shear wave field from a CIRS®
phantom, Figure 1. The characteristics given by the constructor are 80kPa, 45kPa and 14kPa for the
inclusions from left to right. The background, 25kPa is used for the normalised shear elasticity . Our
results, 3.6, 2.2 and 0.54 are in good general agreement with those of the constructor, 3.2, 1.8 and 0.56,
respectively.
Conclusions: The diffuse field approach offers new possibilities of imaging in elastography. It includes
the use of low frame rate imaging devices to track shear waves.
Figure 1: Normalised shear elasticity maps retreived from a diffuse shear wave field of three spherical inclusions of a
CIRS® phantom using a 50Hz B&K scanner. The characteristics given by the constructor are 80kPa, 45kPa
and 14kPa for the inclusions from left to right. The background, 25kPa is used for the normalised shear
elasticity .
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